The gathering of downhole information has been done by the oil well industry for many years. Modern petroleum drilling and production operations demand a great quantity of information relating to the parameters and conditions downhole. Such information typically includes the location and orientation of the wellbore and drilling assembly, earth formation properties, and drilling environment parameters downhole. The collection of information relating to formation properties and conditions downhole is commonly referred to as "logging", and can be performed by several methods.
In conventional wireline logging, a probe or "sonde" having various sensors is lowered into the borehole after some or all of the well has been drilled. The sonde is typically constructed as a hermetically sealed steel cylinder for housing the sensors, and is typically suspended from the end of a long cable or "wireline". The wireline mechanically suspends the sonde and also provides electrical conductors between the sensors (and associated instrumentation within the sonde) and electrical equipment located at the surface of the well. Normally, the cable transports power and control signals to the sonde, and transports information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations adjacent the borehole are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
The sensors used in a wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
For an underground formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well known physical characteristics. For example, one characteristic is that the rock in the formation have space to store petroleum. If the rock in a formation has openings, voids, and spaces in which oil and gas may be stored, it is characterized as "porous". Thus, by determining if the rock is porous, one skilled in the art can determine whether or not the formation has the requisite physical properties to store and yield petroleum. Various wireline sensors may be used to measure formation porosity. Examples include the acoustic sensors described in U.S. Pat. Nos. 3,237,153, 3,312,934, 3,593,255, 4,649,525, 4,718,046, 4,869,349, and 5,069,308.
While wireline logging is useful in assimilating information relating to formations downhole, it nonetheless has certain disadvantages. For example, before the wireline logging tool can be run in the wellbore, the drillstring and bottomhole assembly must first be removed or "tripped" from the borehole, resulting in considerable expense and a loss of drilling time for the driller (who typically is paying daily fees for the rental of drilling equipment). In addition, because wireline tools are unable to collect data during the actual drilling operation, drillers must at times make decisions (such as the direction to drill, etc.) based on limited and possibly insufficient information, or else incur the costs of tripping the drillstring to run a wireline logging tool. Another disadvantage is that because wireline logging occurs a relatively long period after the wellbore is drilled, the accuracy of the wireline measurement can be questionable. As one skilled in the art will understand, the wellbore conditions tend to degrade as drilling muds invade the formation in the vicinity of the wellbore. In addition, the borehole shape may begin to degrade, reducing the accuracy of the measurements.
Because of these limitations associated with wireline logging, there recently has been an increased emphasis on the collection of data during the drilling process itself By collecting and processing data during the drilling process, the necessity of tripping the drilling assembly to insert a wireline logging tool can be eliminated, and the driller can make accurate "real-time" modifications or corrections as needed to optimize drilling performance. For example, the driller may change the weight-on-bit to cause the bottomhole assembly to tend to drill in a particular direction, or, if a steerable bottomhole assembly is used, the driller may operate in the sliding mode to effect source corrections. Moreover, the measurement of formation parameters during drilling, and hopefully before invasion of the formation, increases the usefulness of the measured data. Further, making formation and borehole measurements during drilling can save the additional rig time which otherwise would be required to run a wireline logging tool.
Designs for measuring conditions downhole along with the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while drilling" techniques, or "MWD". Similar techniques, concentrating more on the measurement of formation parameters, have commonly been referred to as "logging while drilling" techniques, or "LWD". While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
The measurement of formation properties during drilling of the well by LWD systems improves the timeliness of measurement data and, consequently, increases the efficiency of drilling operations. Typically, LWD measurements are used to provide information regarding the particular formation in which the borehole is traversing. During the last several years, many in the industry have noted the desirability of an LWD system that could be especially used to detect bed boundaries in a real-time fashion to enable the driller to make directional corrections to stay in the pay zone. Alternatively, the LWD system could be used as part of a "Smart" system to automatically maintain the drill bit in the pay zone. See, e.g. commonly assigned U.S. Pat. No. 5,332,048, the teachings of which are incorporated by reference herein. The assignee has also developed a system which permits the measurement of LWD data at the drill bit to provide an earlier indication of bed boundaries and formation characteristics. See U.S. Pat. No. 5,160,925. The use of an LWD system with these other systems makes it possible to conduct at least certain portions of the drilling process automatically.
Ordinarily, a well is drilled vertically for at least a portion of its depth. The layers or strata that make up the earth's crust are generally substantially horizontal. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. One of the properties of the formation that is commonly logged is its resistivity. LWD tools that are designed to measure the resistivity of the surrounding formation need not be azimuthally focused, as the formation in question surrounds the wellbore and is essentially the same in all directions. Thus the rotation of the LWD tool with the bit has no significant effect on the measured resistivity. For this reason, typical LWD resistivity tools that are adapted for use in vertical wells are azimuthally symmetric and have no azimuthal sensitivity.
In certain applications, however, such as when drilling through formations in which the reservoir boundaries extend vertically, or when drilling from an off-shore platform, it is desirable to drill wells at less of an angle with respect to bed boundaries in the strata. This is often termed "horizontal" drilling. When drilling horizontally, it is desirable to maintain the well bore in the pay zone (the formation which contains hydrocarbons) as much as possible so as to maximize the recovery. This can be difficult since formations may dip or divert. Thus, while attempting to drill and maintain the well bore within a particular formation, the drill bit may approach a bed boundary. As the rotating bit approaches the bed boundary, the bed boundary will be on one side of the bit axis, i.e. in one azimuthal range with respect to the bit axis.
If a near-bit resistivity tool were capable of sensing resistivity values azimuthally, the sensed values could be analyzed to discern the direction of the bed boundary. If the tool were sufficiently sensitive, the approaching bed boundary would be detected in sufficient time to allow the driller to make corrections in accordance with known techniques to avoid exiting the desired formation. Hence, it is desirable to provide an LWD tool that allows azimuthally sensitive resistivity measurements, a tool that is easy to manufacture and assemble and that is sufficiently durable and reliable in the drilling environment.